The invention relates to soldering fluids used in soldering processes. More particularly, this invention relates to the use of a water-rinsable soldering fluid used in a wave-type soldering process. Wave-type soldering processes are commonly used for soldering or tinning electrical circuits, particularly printed circuits. Wave-type soldering processes are described in the book, "Solders and Soldering", by Howard H. Manko, published by McGraw-Hill (1974), the entire disclosure of which is incorporated herein by reference.
The advent of the printed circuit has greatly facilitated the rapid production of accurately connected sophisticated miniature electrical circuits. However, the demands on the soldering device to enhance their efficient use are continuously increasing. In many such devices, oil is used with the solder to provide a better distribution of the solder on the areas to be coated, to aid in removal of the excess solder, to protect the solder from oxidation or discoloration while being cooled, and to keep the hot solder clean by providing a protective coating. In one particular device, solder and soldering oil are pumped together continuously to form a wave through which the bottom of the printed circuit board is passed.
It is well understood that an oil blanket on the solder pot will inhibit dross formation. Dross is an expensive consumer of solder and its abrasive action can also wear out the solder pump. Since dross (the oxides of tin and lead) is formed when molten solder comes in contact with air, there is an advantage in establishing a barrier between the air and the molten solder. Although soldering oils have worked well for this purpose, the oil tends to leave a film on the piece being soldered. The residual oil and other residual components, such as flux, must be removed from the soldered piece. This has typically been done by vapor degreasing or by employing some other oil solvent cleaning process. These have not been wholly satisfactory due to the high cost associated with air pollution control problems. In addition, water-detergent cleaners have been used, but only with moderate success due to incomplete removal of the oily residues from the soldered surfaces and due to problems related to the disposal of the oily wash water. Thus, the industry has long desired a water-rinsable soldering fluid to take the place of soldering oils.
Water-rinsable soldering fluids are available commercially and generally comprise a polyglycol (a polyether) and an antioxidant to help prevent degradation of the polyglycol. A dross scavenger comprising an acidic additive is also frequently added to the soldering fluid. The acidic component, generally a fatty acid, functions to scavenge the dross through chemical reaction. As the solder wave becomes exposed to air during pumping, dross may form on the wave to some degree. When it does, the fatty acid reacts with the lead and tin oxides to form oil dispersible soaps. These soaps remain in suspension for many hours and are removed when the spent oil is removed from the reservoir at set intervals. The water-rinsable soldering fluids provide for easy cleaning of the soldered pieces but suffer the deficiency of not providing good soldering definition by permitting bridging, icicling and excessive solder deposits on the pieces being soldered. Modern-day electrical circuit boards require extremely close tolerances between lines and pieces being soldered and essentially require bridge-free, icicle-free soldering of circuits of narrow line widths frequently as small as 1/2000-inch part. Furthermore, the presence of effective concentrations of organic acids lowers the flash points of the composition to less than 500.degree. F., which is too low for currently desirable high-flash-point soldering fluids.